Call forwarding is a service available in many central offices which allows an incoming call to be directed elsewhere. There are many variations on call forwarding including call forwarding on busy, call forwarding on no answer, call forwarding all calls, etc. Call forwarding is a useful feature, such as when you are going to a meeting but are expecting an important call, you can pick up your phone before you leave for the meeting and have your calls follow you. A big disadvantage with call forwarding is that many people return to their offices but forget they have forwarded their calls elsewhere. As a result, they usually miss a number of calls. Another significant disadvantage is that a user has no way to automatically screen for important calls and select those they want to receive. Call forwarding can be used to send calls to voice mail systems either immediately or after a predetermined number of rings, typically four.
When call forwarding is used in North America from a central office, the service is activated by entering in 72# and the number where the calls are to be forwarded to. To cancel the feature you enter in 73#.
Central offices now offer a feature whereby calls that are forwarded to another telephone number, an answering service, or a telemessage provider. This feature is described in detail in the document Bellcore GR-1512-CORE, Issue 2, October 1994, A Module of LSSGR. Currently a user receives a special reminder tone, 20 Hz for 400-600 millisecond, and then can place the telephone line in the off-hook position. At this point the user can monitor the call being forwarded while the calling party is unaware of their presence. A switch hook flash signal can be sent from the user if they want to connect with the calling party, which will then establish a three way call between the calling party, the call screening customer, and the remote party. Otherwise the user can hang up, which terminates the screening without the knowledge of the calling party.
Accordingly, a need exists for automatic answering in response to a special ring tone to provide automatic call screening.